t)U GEOLOGY. 



diagmmmatically in Fig. 37, which illustrates both the diminution 

 in area which the island has suffered, and the increase in the angle of 

 its slopes. Immediately about it, at the stage represented by aa, 

 Fig. 37, there is a narrow marginal platform, or submerged terrace, 

 in place of the land area which has been worn away at or just below 

 the level of the sea. Long successions of rains working in the same 

 way will give the island steeper slopes, a smaller area, and a wider 

 marginal terrace. Successive stages are shown by the lines ?)6 and cc, 

 Fig. 37. 



If rain falls on such an island until it completes the work which 

 it is possible for running water to do, the island will be reduced es- 

 sentially to the level of the sea, and in its place there will be a plain, 

 the area of which will be equal to that of the original island. Its central 

 point will be at the level of the sea, and its borders a trifling distance 

 below it (Fig. 38). The island is gone, and in its place there is a plain 



Fig. 38. — Diagram to illustrate the final effect of rain erosion under the conditions 

 specified in the text. The diagram expresses the final result of the processes sug- 

 gested by Fig. 37. 



as low as running water can wear it. Other agencies might come in to 

 defeat the result just outhned, but if the island did not rise or sink 

 after its formation, rain falling upon it would, under the conditions 

 specified, finally bring about the result which has been sketched. The 

 plain (Fig. 38) which succeeds the island is a base-level of erosion, though 

 this term is also used in other ways. Under these conditions the slope 

 of the land would remain convex at all stages, but the convex erosion 

 profile of the land would meet a nearly straight line just below sea- 

 level. The relative lengths of these two elements of the profile, the 

 curve above and the straight fine below, vary as erosion progresses, 

 the convex portion becoming shorter and the other longer. The two 

 parts of the profile taken together are concave upward at the lower 

 end all the time, and for a greater distance from its lower end in all 

 the advanced stages of erosion (Fig. 37). 



In the destruction of the land under these conditions neither valleys 

 nor hills would he developed, nor would the topography of the land be 



